The present disclosure relates to a method for crystallizing a semiconductor thin film by irradiation of an energy beam.
In flat displays such as liquid crystal displays and organic EL displays using organic electroluminescent elements, thin film transistors (TFT) are used as a switching element for the active matrix display of a plurality of pixels. The thin film transistor includes TFT using polysilicon (poly-Si) as an active region (polysilicon TFT) and TFT using amorphous silicon (amorphous Si) as an active region (amorphous silicon TFT).
Of these, the polysilicon TFT has the feature that it is about 10 times to 100 times greater than the amorphous silicon TFT with respect to the carrier mobility, with reduced degradation of on current. In this sense, the polysilicon TFT not only has very excellent characteristics for use as a switching element of such displays as mentioned above, but also has attracted attention for use as switching elements constituting various types of logic circuits (e.g. domino logic circuits, CMOS transmission gate circuits and the like) and multiplexers, EPROM, EEPROM, CCD and RAM using the logic circuits).
For a manufacturing technique of such a polysilicon TFT, there has been developed a so-called low temperature polysilicon process using a low temperature process of approximately 600° C. or below, thereby realizing low costs of a substrate. In the low temperature polysilicon process, a pulse laser crystallization technique has been in wide use wherein an amorphous silicon film is crystallized by use of a pulse laser whose oscillation time is very short. The pulse laser crystallization technique is a technique wherein a silicon thin film on a substrate is irradiated with a high-output pulse laser beam to instantaneously melt the film and such a property of silicon being crystallized during the course of solidification is utilized.
For instance, in the low temperature polysilicon process using an excimer laser, a line-shaped laser beam is shifted little by little for pulse irradiation on an amorphous silicon film while overlapping irradiation on most of portions of the film, under which 10 to 20 cycles of laser beam irradiation on the same portion are repeated. The polycrystal whose crystal size is uniform may be obtained on the entire surface of active region by this process. Moreover, a method of controlling the position of crystal grains using crystallization according to an SLS (sequential lateral solidification) procedure has been proposed. In addition, there has also been proposed, for example, a method of controlling the position of crystal grains by spacially modulating a phase of an excimer laser beam through a phase shift mask to have the irradiating laser beam had an energy density gradient (see “Surface Science 21”, 2000, vol. 21, No. 5 pp. 278-287).
Aside from those methods using a line-shaped laser beam as set out above, there has been proposed a method for arranging relatively small-sized crystals by explosive crystallization with use of a spot beam laser such as of Ar gas.
In recent years, in such flat panel displays as set forth above, a high frame-rate liquid crystal display is being developed for the purpose of further improving a moving image characteristic and a contact characteristic. New types of displays such as a self-emitting display such as an organic EL display are now under development. This entails the demand for development of TFT for use as a switching element applicable to these displays, which undergoes little characteristic degradation upon abrupt passage of a great current and is small in characteristic variation among switching elements.
In this connection, although the polysilicon TFT obtained by the prior-art low temperature polysilicon process is very advantageous in that it has the likelihood of passing a relative great current, has a great carrier mobility and is small in characteristic degradation, variations in element-to-element characteristics, especially, an initial threshold voltage and an on current, are greater than those of the amorphous silicon TFT. The characteristic variation among the polysilicon TFT elements is a factor of causing uneven brightness in the display using the polysilicon TFT as a switching element.
The element-to-element variation in characteristics of the polysilicon TFT depends on the variation in number of grain boundaries existing in a channel portion of the polysilicon TFT in a channel direction (i.e. a direction along which electrons flow). Accordingly, within a range where the number of crystal boundaries is small, a slight difference in number of grain boundaries brings about a great variation of TFT elements. On the other hand, as the number of grain boundaries increases, a variation of TFT elements is suppressed low even if the number of grain boundaries in the channel portion differs more or less. In order to suppress the characteristic variation in the polysilicon TFTs to a low level, importance is placed on the formation of a polysilicon film wherein uniformly shaped, relatively small-sized crystals are regularly aligned.
However, an excimer laser widely employed in the pulse laser crystallization technique is a kind of gas laser, for which interpulse energy stability is low. Although polycrystals whose crystal size is uniform can be obtained by 10 to 20 cycles of laser beam irradiation on the same portion as stated hereinabove, the uniformity of the resulting crystal size is unsatisfactory. Additionally, the excimer laser is high in unit cost along with high running costs necessary for change of a laser tube (oscillator). The necessity of repetition of irradiation by about several tens of cycles lowers throughput, with the attendant problem that manufacturing costs of a product cannot be lowered.
The problem on the uniformity of crystal size being unsatisfactory is likewise experienced in such a method using a phase shift mask as discussed in “Surface Science 21”, 2000, vol. 21, No. 5 pp. 278-287. This method has additional problems such as of high fabrication costs of the phase shift mask and a difficulty in making a large-sized substrate.
An explosive crystallization method using a spot beam laser such as of Ar gas is a recrystallization method using solid phase transition, for which the resulting crystals are poor in quality and it is difficult to obtain a satisfactory carrier mobility.
It is desirable to provide a method for crystallizing a semiconductor thin film wherein crystal grains having a good shape accuracy are regularly aligned, thereby enabling a crystal region of good accuracy exhibiting a high carrier mobility to be formed.